1. Field of the Invention
The present invention relates to an improved pressure-operated switch for a high-voltage interrupting module. More specifically, the present invention relates to an improvement of the switches disclosed in commonly assigned U.S. Pat. Nos. 4,342,978 issued Aug. 3, 1982 in the name of Meister, and 4,370,531 issued Jan. 25, 1983 in the name of Tobin, and in the following commonly assigned U.S. patent application: Ser. No. 179,367 filed Aug. 18, 1980 (now abandoned in favor of continuation application Ser. No. 550,201, filed Nov. 9, 1983) in the name of Jarosz and Panas; Ser. No. 179,366 filed Aug. 18, 1980 (now abandoned in favor of continuation application Ser. No. 539,396, filed Oct. 6, 1983) in the name of O'Leary, and Ser. No. 437,925 which issued Jan. 24, 1984 as U.S. Pat. No. 4,427,963, and Ser. No. 437,926, both filed Nov. 1, 1982 in the names of Jarosz and Panas.
2. Prior Art
The above-noted commonly assigned patents relate to various aspects of a pressure-operated switch and to a high-voltage interrupting module containing the switch. The switch may include a pair of contacts which are normally electrically interconnected, for example, by direct abutment therebetween or, preferably, by interconnecting them with a shearable or tearable metallic disc or membrane. In preferred embodiments of the switch, one contact is stationary, while the other is movable, although both may be movable. The contacts are separable by relative movement along a fixed line of direction to open a gap therebetween, thereby opening the switch. One of the contacts, preferably the stationary contact, contains a bore which, in conjunction with a piston or trailer positioned between the movable contact and the bore, defines a closed chamber. The chamber houses a power cartridge or similar pressure-generating device.
The switch may be in electrical shunt with a fuse, a fusible element which, as well as the switch, preferably reside within a common housing. When the switch is closed (i.e., when the contacts thereof are electrically interconnected), the resistance of the current path through the switch is much lower than resistance of the current path through the fusible element, and, accordingly, a majority of the current flowing through the module flows through the switch. Thus, the module has a very high continuous current rating. Upon opening the switch, the contacts separate and current is rapidly commutated from the switch to the fusible element where it is interrupted. Separation of the contacts is achieved by igniting the power cartridge, which evolves high pressure within the chamber. This high pressure acts against the piston and the forces produced thereby rapidly drive the piston and the movable contact away from the stationary contact, which shears the disc to break the normal electrical interconnection and open the switch. The power cartridge may be ignited in response to a trip signal produced by apparatus which senses a fault current or other overcurrent in a circuit in which the interrupting module is connected for protection thereof. Such trip-signal-producing apparatus may be that which is disclosed in commonly assigned U.S. patent application, Ser. Nos. 506,942; 506,943; and 506,944, all filed June 22, 1983 in the name of Ruta.
In specific embodiments of the switch described in the above patents and patent applications, a second stationary contact is included. When the switch is closed, the movable contact and the second stationary contact are electrically interconnected with a second shearable disc. When the power cartridge is ignited, movement of the movable contact also shears the second disc. As the movable contact moves away from the first stationary contact, it is telescoped into a bore formed in the second stationary contact. This bore may be lined with an insulative sleeve and the movable contact may be covered with an insulative sleeve, so that such telescoping results in the formation of a second gap between the movable contact and the second stationary contact.
The movable contact moves rapidly away from the first stationary contact through a passageway in an insulative liner. The piston also enters the passageway in the liner to physically isolate the moving contact and the second stationary contact from the ignition products of the power cartridge. This isolation prevents or suppresses the formation of any arc between the separating contacts and between the stationary contacts. In preferred embodiments of the switch, the stationary contacts and the liner are engageably surrounded, and have their relative positions fixed, by an insulative housing, which maintains the stationary contacts and the liner end-to-end with the bores and the passageway axially aligned.
Tests of earlier versions of the switch (such as those disclosed in the '978 and '531 patents and in the '367 and '366 applications) showed that, after the piston entered the liner, some of the ignition products of the power cartridge might, in some cases, flow along the liner-housing interface. Such flow, it was noted, might result in internal flashover of the open switch, i.e., undesired conduction there within. It is one object of the present invention to eliminate this problem.
Additionally, it was noted that high electrical stress, which might also result in internal flashover of the open switch, could occur between the regions of abutment between the liner and the stationary contacts after the switch had opened. Another object of the present invention is the elimination of such flashover.